The present invention relates to a developing device and an image forming apparatus having the developing device.
In a conventional image forming apparatus of an electro-photography type such as a printer, a facsimile, a copier, and the likes, a charging roller uniformly charges a surface of a photosensitive drum, and an exposure device exposes the surface of the photosensitive drum to form a static latent image thereon. After a developing device develops the static latent image to form a toner image on the photosensitive drum, a transfer roller transfers the toner image to a sheet. Then, a fixing device fixes the toner image to the sheet, thereby forming an image on the sheet.
In the conventional developing device, a developing roller is disposed to be freely rotatable and abut against the photosensitive drum for forming the toner image. Further, a toner supply roller is disposed to be freely rotatable and abut against the developing roller. A developing blade is disposed to contact with a surface of the developing roller at a distal end portion thereof. When toner as developer retained in a toner cartridge is supplied to the conventional developing device, the toner supply roller supplies toner thus supplied to the developing roller, and scrapes off toner remaining on the developing roller without being developed, i.e., undeveloped toner.
In the conventional developing device, when toner is supplied to the developing roller, the developing blade forms a thin layer of toner while the developing roller rotates. Further, toner is charged with a specific polarity, thereby forming a toner layer on the developing roller. Then, toner of the toner layer is transported to a developing portion between the developing roller and the photosensitive drum, and is attached to the static latent image on the photosensitive drum, thereby visualizing the static latent image and forming the toner image. (Refer to Patent Reference)    Patent Reference Japanese Patent Publication No. 10-83116
In the conventional printer, when the toner supply roller is not able to sufficiently scrape off undeveloped toner remaining in a recess portion of surface undulation of the developing roller, an afterimage may occur. To this end, the developing roller is configured to have less undulation. In this case, the developing roller is not capable of transporting a sufficient amount of toner, thereby reducing an image density. In other words, there is a tradeoff relation between prevention of an afterimage and obtaining a sufficient image density.